I would start with social foxtrot and cha cha the first week. Cha cha is easier than rumba for beginners but if you stick with a basic routine (4 basic, 4 shoulder to shoulder, 3 new york, spot turn) you can convert this to rumba a bit later on and they get an extra dance for free!

Similarly a simple social foxtrot would be a good starting point to speed up into quickstep.

Most beginners will expect to learn waltz but generally find it harder than they thought. I find it is the closing of the feet on 3 that causes the most problem as they get confused which foot to move next. Emphasise that they should use alternate feet just as when walking and make sure they change their weight when they close.

As others have mentioned the men may need treating with kid gloves. They can get very frustrated that they cannot manage a simple thing like dancing!